The present invention relates to pulp washing systems and, particularly, to a method and apparatus for periodically cleaning screens in a drum pulp washing system.
Drum washers are used in various fiberline process stages to displace high solids filtrate from pulp stock. A drum displacement washer, such as the Drum Displacer® (DD) Washer supplied by Andritz Oy of Finland, is typically a pressurized multistage washer. A DD Washer includes a rotating drum having rows of trays filled with pulp as wash liquid is sprayed on the trays. The wash liquid passes through the pulp in the tray and removes chemicals and filtrate from the pulp. Screens in the bottom of the trays allow the wash liquid to pass through the trays and retain the pulp in the trays.
The screens may clog and corrode due to the pulp fibers retained by the screens and the flow of wash liquid, chemicals and filtrate through the screens. The screens are periodically cleaned to open clogged screen slots and remove corrosion on the screens. The screens are cleaned while the drum is stopped and the washer undergoes maintenance.
Cleaning both sides of conventional screens typically requires the screens to be removed from the tray by sliding the screen from the end of the tray. Accessing the ends of the trays to remove the screens is difficult due to the housing encasing the drum. Cleaning of the screens is typically a time consuming operation because of the difficulty in removing and replacing screens from the axial ends of the trays. Alternatively, the screens are partially cleaned by spray washing the upper surface of the screens. This alternative spray washing technique does not spray the bottom surfaces of the screens. This alternative spray washing technique is performed quickly and, thus, has advantages over cleaning by removing the screens. However, the alternative spray washing technique does not clean the bottom of the screens. Accordingly, there is a long felt need for an improved system and method to clean both sides of the screens in the trays of a pulp washer.